S1) Skeletal Muscle Morphology, Structure & Mechanics

Question 1

What are the 3 types of muscle cells?

Answer 1

• Skeletal
• Cardiac
• Smooth

Question 2

State the structure and function of skeletal muscle

Answer 2

• Structure: striated muscle due to the ordered arrangement of the myofibrillar apparatus producing a characteristic banding pattern
• Function: permit movement through voluntary contractions

Question 3

Describe the role of skeletal muscle in heat generation

Answer 3

• Approximately 80% of chemical energy is lost as heat as a by-product of muscle activity
• This inefficiency is used by the physiological process of shivering to raise core body temperature by the involuntary activation of skeletal muscle

Question 4

Describe the numerical distribution of skeletal muscles in the body

Answer 4

• Approximately 640 skeletal muscles
• Most of which are present as bilateral pairs

Question 5

The most common arrangement of skeletal muscle is for the fibres to run parallel to the force-generating axis.

Identify the three types of parallel muscles

Answer 5

Question 6

Describe the structure of strap muscles

Answer 6

Strap muscles are shaped like a strap with the fibres running longitudinally, parallel to the direction of contraction e.g. sartorius

Question 7

Describe the structure of fusiform muscles

Answer 7

Fusiform muscles are cylindrical and wider in the centre while tapering off at the ends e.g. biceps brachii

Question 8

Describe the structure of fan-shaped muscles

Answer 8

Fan shaped muscles have fibres that converge at one end e.g. pectoralis major

Question 9

What are pennate muscles?

Answer 9

Pennate muscles are muscles which 1/more aponeuroses running through the muscle body from the tendon and fascicles which attach to these aponeuroses obliquely to the direction of movement

Question 10

Identify and describe the different kinds of pennate muscles

Answer 10

• Unipennate where all the fascicles are on the same side as the tendon
• Bipennate where fascicles are on both sides of a central tendon
• Multipennate where a central tendon branches

Question 11

What is a pennation angle?

Answer 11

The pennation angle is the angle between a fascicle’s orientation and the tendon axis

Question 12

What are circular muscles?

Answer 12

Circular muscles are muscles where fibres form concentric rings around a sphincter or opening by attaching to skin, ligaments and fascia of other muscles rather than to bone

E.g. orbicularis oculi around the eye, orbicularis oris around the mouth

Question 13

What are agonist and antagonist muscles?

Answer 13

• An agonist is the prime muscle(s) responsible for a particular movement
• An antagonist is the muscle(s) which opposes this movement

Question 14

What are synergists?

Answer 14

Synergists are muscles which act to assist the prime mover as they cannot act alone to perform the movement e.g. brachioradialis and pronator teres in elbow flexion

Question 15

What are neutralisers?

Answer 15

Neutralisers are muscles which prevent the unwanted actions that an agonist can perform

E.g. rotator cuff muscles stabilise the glenohumeral joint whilst biceps, (whose long head acts to cause shoulder flexion) cause elbow flexion

Question 16

What are fixators?

Answer 16

Fixators are muscles which act to hold a body part (proximal joint) immobile whilst another body part (distal joint) is moving

E.g. elbow flexion – fixators are the muscles that stabilise the scapula and those that stabilise the shoulder joint

Question 17

Distinguish between neutraliser and fixator muscles

Answer 17

• Neutralisers prevent the unwanted actions of a muscle
• Fixators stabilise a joint

Question 18

What is muscle contraction?

Answer 18

Muscle contraction refers to the active cycling of cross-bridges between the actin thin filaments and the myosin thick filaments within the sarcomeres

Question 19

What is isotonic contraction?

Answer 19

Isotonic contraction is where the tension within the muscle remains constant and the length changes

Question 20

Identify and describe the 2 types of isotonic contraction

Answer 20

• Concentric contraction: muscle shortens e.g. biceps during elbow flexion
• Eccentric contraction: muscle lengthens e.g. quadriceps during walking

Question 21

What is isometric contraction?

Answer 21

Isometric contractions is where the length of the muscle stays contant but tension is generated e.g. holding a weight in a fixed position, or trying to push a brick wall

Question 22

What is passive stretch?

Answer 22

Passive stretch is the lengthening of the muscle muscle in a passive state i.e. not being stimulated to contract

E.g. pull felt in the hamstrings whilst touching the toes

Question 23

The protein titin is now known to be important in the contraction of striated muscle tissues.

What is its role in passive stretch?

Answer 23

• Titin connects the Z line to the M line in the sarcomere and contributes to force transmission at the Z line and resting tension in the I band region
• Titin limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle

Question 24

Briefly, describe the role of motor neurons in the skeletal muscle

Answer 24

Each skeletal muscle is supplied by a number of motor neurons which stimulate the muscle fibres to contract

Question 25

What type of motor neurons innervate the skeletal muscle fibres?

Answer 25

α-motor neurons

Question 26

Where are the cell bodies of alpha motor neurones found?

Answer 26

- **Ventral horn of the spinal cord** for muscles of the limbs and trunk - **Motor nuclei of the brainstem** for the muscles of the head and face

Question 27

What is a neuromuscular junction?

Answer 27

A **neuromuscular junction** is a specialised synapse between an a motor neurone and a skeletal muscle fibre

Question 28

In 6 steps, outline the events which occur at a neuromuscular junction

Answer 28

⇒ **Action potential** arrives at the synapse ⇒ Voltage-gated **Ca²⁺ channels** open ⇒ Increased [Ca²⁺] causes **release of ACh** from vesicles into synaptic cleft ⇒ ACh activates nicotinic ACh **receptors** in the sarcolemma ⇒ Influx of Na⁺ **depolarises** sarcolemma (local) ⇒ Voltage sensitive Na⁺ channels are activated to produce general depolarisation

Question 29

What is a motor unit?

Answer 29

- A **motor unit** is an α-motor neuron and the group of individual muscle fibres that it innervates - Any single muscle fibre is innervated by only one α-motor neuron, but each α-motor neuron can innervate a number of different muscle fibres

Question 30

The number of muscle fibres in a motor unit varies between different muscles. Illustrate this

Answer 30

- Muscles that perform precise **fine movements** have less muscle fibres each unit motor unit - Powerful muscles with **crude movements** have far more muscle fibres in each motor unit

Question 31

What is muscle tone?

Answer 31

**Muscle tone** is the continuous and passive partial contraction of the muscles, or the muscle's resistance to passive stretch during resting state

Question 32

All muscle has some degree of baseline tone. What contributes to this?

Answer 32

- Elasticity of the muscle tissue - Low levels of motor neuron activity

Question 33

Why is hypotonia?

Answer 33

**Hypotonia** is a lack of skeletal muscle tone and results from damage to the motor cortex, cerebellum or spinal cord

Question 34

Skeletal muscle tone is controlled by motor control centres in the brainstem. Describe this control

Answer 34

The **locus coeruleus**, containing noradrenergic cells, projects ascending axons to spinal motor neurons where it facilitates muscle tone

Question 35

The contractile force produced by a muscle depends on two factors. Identify and describe them

Answer 35

- **Size principle:** slow type I fibres are recruited first followed by those containing mostly fast IIa fibres and then those containing fast IIX fibres - **Rate code:** the frequency at which the muscle fibres are stimulated by their α-motor neuron

Question 36

Identify the sources of energy for muscle contraction

Answer 36

- **ATP** stores (very little) - Stores of **creatine phosphate** (short term) - ATP from **glycolysis** and **oxidative metabolism** (long-lasting)

Question 37

State an advantage and disadvantage of glycolysis

Answer 37

- **Advantage**: function under anaerobic conditions so not reliant on the blood supplying oxygen at the rate required for maximal contraction - **Disadvantage**: lactate is produced from pyruvate by LDH and accumulates

Question 38

What are the effects of the accumulation of lactate?

Answer 38

**Lactate accumulation** is associated with acidification of the muscle cell environment leading to cramps and muscle fatigue

Question 39

Describe contraction in Type I muscle fibres

Answer 39

- **Slow muscle fibres** express the type I myosin heavy chain - They contract relatively **slowly**, produce low amounts of **force** and are extremely resistant to **fatigue** (high mitochondria content, oxidative metabolism)

Question 40

Describe contraction in Type II muscle fibres

Answer 40

- **Fast muscle fibres** express the type II myosin heavy chain - They contract relatively **quickly** and produce large amounts of **force**

Question 41

Identify and describe the two different types of Type II muscle fibres

Answer 41

- The **fast IIX muscle fibres** rely on glycolytic metabolism, produce great amounts of force and fatigue rapidly - The **fast IIA muscle fibres** are an intermediate form between Type I oxidative fibres and fast IIX glycolytic muscle fibres

Question 42

What is malignant hyperthermia?

Answer 42

**Malignant hyperthermia** is a disease that causes a fast rise in body temperature and severe muscle contractions when triggered by some volatile anaesthetic agents

Question 43

What is the most common cause for malignant hyperthermia?

Answer 43

Polymorphism in the ryanodine receptor (genetic cause)

Question 44

In 4 steps, explain how malignant hyperthermia presents with an uncontrolled increase in oxidative metabolism

Answer 44

⇒ Ryanodine receptor is activated by anaesthetic agents ⇒ Massive increase in [Ca²⁺]_i from intracellular stores ⇒ SERCA must function at a dramatically increased rate ⇒ Excessive heat production

Question 45

How can malignant hyperthemia be treated?

Answer 45

**Dantrolene** – a muscle relaxant which antagonises the ryanodine receptor